Process for the rational exchange of heat in counter-current heat exchanges where the exchanges are unbalanced

ABSTRACT

The coupling of quantities of heat by exchange between two fluids is obtained by dividing the hot fluid into two streams at the entrance of a heat exchanger; the first stream is introduced into the high temperature portion of the exchanger and is then passed to a turbine for reduction in pressure while producing work energy; the second stream being introduced into a turbine for reduction in pressure while producing work energy and is then passed to the heat exchanger at substantially the temperature at which the first stream leaves the heat exchanger. The excess heat which is available in the hot fluid and cannot be stored for reheating the other fluid is thus converted into work energy.

United States Patent Sterlini [151 3,667,230 [451 June6, 1972 [54] PROCESS FOR THE RATIONAL EXCHANGE OF HEAT IN COUNTER- CURRENT HEAT EXCHANGES WHERE THE EXCHANGES ARE UNBALANCED Primary ExaminerManin P. Schwadron Assistant Examiner-Allen M. Ostrager AttrneyPierce, Scheffler 84 Parker [57] ABSTRACT [72] Jacques Sterlini Paris France The coupling of quantities of heat by exchange between two [73] Assignee; Compagnie E|ectl-o Mecanique, Paris, fluids is obtained by dividing the hot fluid into two streams at France the entrance of a heat exchanger; the first stream is introduced into the high temperature portion of the exchanger and is then Flledl 26, 1971 passed to a turbine for reduction in pressure while producing [21 App No: 109 894 work energy; the second stream being introduced into a turbine for reduction in pressure while producing work energy and is then passed to the heat exchanger at substantially the 2] l /73, temperature at which the first stream leaves the heat 6 /108 exchanger. The excess heat which is available in the hot fluid [51 Int. Cl ..F0ld 13/02 and cannot be stored for reheating the other fluid is thus con- [58] Field of Search r.- ..60/65, 70, 73, 108, 69, verted into work energy.

[56] References Cited 7 Claims, 4 Drawing Figures UNITED STATES PATENTS 3,100,967 8/1963 Brunner ..60/lO5 X WWWW PATENTEDJHH 6 I912 SHEEI 10? 2 g @E ma QGF H a??? PROCESS FOR THE RATIONAL EXCHANGE OF HEAT IN COUNTER-CURRENT HEAT EXCHANGES WHERE THE EXCHANGES ARE UNBALANCED It frequently happens in practice that one may have to operate heat exchangers where the amounts of heat to be exchanged cannot be coupled at all levels of temperature; thus, there result important losses of utilizable energy. I One encounters this situation in counter-current heat exchangers where the exchanges between the primary fluid to be reheated and the secondary fluid which is cooled while reheating the primary fluid set in action different variations in enthalpy of the fluid as a function of the temperature. This is especially the case in changes having one or the other of the following characteristics:

1. The primary and secondary fluids have different chemical characteristics.

2. The primary and secondary fluids are of the same nature, but have different mass-flows per second;

3. The primary and secondary fluids are of the same nature, and their mass-flows are equal, but their apparent specific heats at given temperatures depend on their pressures.

The last case assumes great practical importance because one frequently encounters it in the thermodynamic cycles of thermal machines employing counter-current heat exchangers.

The object of the present invention is a process applicable to counter-current heat exchangers when the secondary fluid, for supplying heat,.is compressible and is capable of yielding during its cooling isobar (with the exception of losses of pressure in the exchanger) quantities of heat which, in certain temperature ranges are greater than those necessary for reheating the primary fluid. This circumstance is clearly encountered most frequently in. practice.

The process according to the invention which has for its object to render rational, in the thermodynamic sense, the exchanges between primary and secondary fluids under the conditions defined above, is characterized in that the coupling of quantities of heat by an exchanger is obtained by dividing at the entrance to the heat exchanger, the secondary fluid, into two streams evolving in parallel after the following process:

The first stream of the secondary fluid begins by exchanging at constant pressure, with the exception of losses of pressure in the exchanger, its heat with the primary fluid which circulates counter-current in the exchanger, then it is expanded In order to better understand the invention a flow sheet in which the principle is employed is described below with reference to the annexed drawings and is indicative and not lirnitative of the invention.

In the drawings:

FIG. 1 and FIG. 2 are entropic diagrams of the primary and secondary fluids respectively;

FIG. 3 is a schematic view of an installation employing the principles of the invention; and

FIG. 4 represents the thermodynamic evolutions of the primary and secondary fluids in the first stages of the installation.

In the diagram of FIG. 1, the ordinate represents the temperature -T and the abscissa represents the entropy S,. This diagram has been established for a mass of the primary fluid equal to the mass flow per second of that fluid which goes through the exchanger; for simplification there is shown only two isobars P and P, adjacent one another, between which is a line representative of the state of the primary fluid during passage through the exchanger; this line which is represented by a dotted line from point a located on isobar P at temperature T is representative of the state at the beginning of the exchanger and ending at point I: located on Isobar P at temperature T is representative of the state at the end of the exchanger.

The entropic diagram of FIG. 2 has been established for a mass of the secondary fluid equal to the mass flow per second of that stream which is going to be passed through the exchanger; the entropy of the secondary fluid has been noted at S there is represented on this diagram only the isobar P corresponding to the pressure at the entrance to the exchanger; the point representative of the state of the secondary fluid at the entrance to the exchanger is the point r: (pressure P temperature T the difference of temperature at the entrance of the exchanger thus being T T',).

The units chosen on these two diagrams being the same, the slopes of the isobar P are supposed to be smaller, for all the relating temperatures of the exchange,than the corresponding slopes of theline representing the state of the primary fluid.

In FIG. 4 there is shown side by side the entropic diagrams of the primary fluid (diagram T, 8 and the secondary fluid (diagram T, S the axes of the ordinates for temperature being common to the two diagrams.

adiabatically in a turbine while furnishing work energy; there is then a thermodynamic state characterized by a final pressure and a final temperature T The second stream is at first expanded adiabatically in a turbine while furnishing work energy until a pressure in excess of the final pressure attained by the first stream by a quantity equal to the loss of pressure which will occur in the operation which follows: an exchange of heat with the primary fluid to the final temperature previously attained by the first stream.

The first and second streams are thus in the same thermodynamic state and can be remixed without irreversibility.

The quantity of total heat involved in the course of the exchanges for the first and second streams passing through a certain temperature range should be equal to the quantity of heat necessary for reheating the primary fluid passing through the same temperature range.

This condition is not sufficient to quantitatively define all the operations that have been described, which can be restored by a choice of mass fractionation of the two streams and the final state of the two streams combined. These choices are entirely fixed since one imposes the two conditions stated below:

for one part, the temperature of the first stream at the end of the exchange ought to be substantially equal to the temperature at the end of the expansion of the second stream;

for the other part, the differences of temperatures of the exchanges both initial and final, and that for the two streams, ought to be imposed (one is able,for example to take them as equal).

In the schematic installation for putting the process into operation according to the representation shown in FIG. 3, the secondary fluid is introduced at A, it being divided into two streams at B.

The first stream enters the exchanger 1 at a pressure P temperature T (see FIG. 4) and exits at C at pressure P' (different from P by an amount corresponding to the loss of pressure in that exchanger) and at the temperature T this evolution corresponding to the dotted line bc in FIG. 4; during this operation it has exchanged its quantity of heat brought into action during this evolution, with the primary fluid ascending in the exchanger from D (FIG. 3) (temperature T' to E (temperature 1",); this evolution corresponding to the distance d e on the entropic diagram of the primary fluid (FIG. 4); the first stream is then passed into the turbine 2 and exits (FIG. 3) at F at a pressure P';,, temperature T, (evolution c f in FIG. 4).

The second stream on this side passes through turbine 3 and exits at G (FIG. 3) at a temperature substantially equal to said temperature T and at a pressure P (evolution b g in FIG. 4), entering the exchanger and exiting at F at the pressure P;,, temperature T.,, after having exchanged heat which occurs the length of the evolution g f of FIG. 4 with the primary fluid ascending from H (FIG. 3) (temperature T,;,) towards D.

In H6. 3 a second operation of the same nature is repeated for obtaining exchanges at lower pressures.

I claim: v

1. Process for obtaining rational exchanges of heat in counter-current exchangers when the hot fluid is compressible and capable of yielding during its cooling, greater quantities of heat than those which are able to be stored by the fluid being reheated, the difference being converted into work energy, characterized in that the coupling of the quantities of heat in the exchanger is obtained by dividing the hot fluid into two streams at the entrance of the exchanger, the first stream passingdirectly into the exchanger at the high temperature part of the exchanger and then being expanded externally of the heat exchanger in a turbine while producing work energy, the second stream being first passed through a turbine, while producing work energy and thence being introduced into the exchanger, leaving it at the temperature at which the first stream leaves the turbine.

2. A process according to claim 1 characterized in that the two streams have, at the end of the operations, the same temperature and pressure and can be combined without loss.

3. Process according to claim 1 characterized in that the quantity of heat given up altogether by the first and second streams of the hot fluid is equal to that necessary for reheating, at the same total difi'erence of temperature, the fluid to be reheated ascending in counter-current.

4. A process as claimed in claim 1 wherein the first stream leaves the turbine at the same temperature as the second stream as it leaves the exchanger and further comprising combining said two streams and then dividing said combined pansion element for producing work energy,means for passing I a first portion of the hotter fluid through said first counter-current heat exchanger element and then through said second expansion element, means for passing a second portion of the hotter fluid through said first expansion element and then through said second counter-current heat exchanger element and means for passing the cooler fluid to be heated countercurrently through said second counter-current heat exchanger element and then through said first counter-current heat exchanger element.

6. Apparatus as claimed in claim 5 wherein said first and second expansion elements for producing work energy comprise turbines.

7. Apparatus as claimed in claim 5 wherein said countercurrent heat exchangers and said expansion elements cool the hotter fluid to such an extent that the first portion thereof as it leaves said second expansion element and the second portion thereof as it leaves said second counter-current heat exchanger have the same temperature and further comprising means for combining the first and second portions. 

1. Process for obtaining rational exchanges of heat in countercurrent exchangers when the hot fluid is compressible and capable of yielding during its cooling, greater quantities of heat than those which are able to be stored by the fluid being reheated, the difference being converted into work energy, characterized in that the coupling of the quantities of heat in the exchanger is obtained by dividing the hot fluid into two streams at the entrance of the exchanger, the first stream passing directly into the exchanger at the high temperature part of the exchanger and then being expanded externally of the heat exchanger in a turbine while producing work energy, the second stream being first passed through a turbine, while producing work energy and thence being introduced into the exchanger, leaving it at the temperature at which the first stream leaves the turbine.
 2. A process according to claim 1 characterized in that the two streams have, at the end of the operations, the same temperature and pressure and can be combined without loss.
 3. Process according to claim 1 characterized in that the quantity of heat given up altogether by the first and second streams of the hot fluid is equal to that necessary for reheating, at the same total difference of temperature, the fluid to be reheated ascending in counter-current.
 4. A process as claimed in claim 1 wherein the first stream leaves the turbine at the same temperature as the second stream as it leaves the exchanger and further comprising combining said two streams and then dividing said combined streams into a first and second stream and repeating said steps for further heat exchange with the fluid being reheated, and for further conversion of excess heat into work energy.
 5. Heat exchanger apparatus for two fluids, the hotter fluid being compressible and having more heat than is required to heat the cooler fluid to the desired temperature, said apparatus comprising at least a first and a second counter-current heat exchanger element, at least a first and a second expansion element for producing work energy,Means for passing a first portion of the hotter fluid through said first counter-current heat exchanger element and then through said second expansion element, means for passing a second portion of the hotter fluid through said first expansion element and then through said second counter-current heat exchanger element and means for passing the cooler fluid to be heated counter-currently through said second counter-current heat exchanger element and then through said first counter-current heat exchanger element.
 6. Apparatus as claimed in claim 5 wherein said first and second expansion elements for producing work energy comprise turbines.
 7. Apparatus as claimed in claim 5 wherein said counter-current heat exchangers and said expansion elements cool the hotter fluid to such an extent that the first portion thereof as it leaves said second expansion element and the second portion thereof as it leaves said second counter-current heat exchanger have the same temperature and further comprising means for combining the first and second portions. 